49 2 RELATION TO ENVIRONMENT. 



5. Provision for fruiting. For most of the aquatic flowering 

 plants it is necessary that the flowers during pollination shall be 

 above the surface of the water. Some curious provisions are 

 made for this, as in the Vallisneria spiralis. The slender, grass- 

 like leaves of this plant are submerged. The stem which bears 

 the pistillate flowers elongates rapidly at time of flowering and 

 lifts the flower to the surface. At the same time the staminate 

 flower-buds, which are borne on short stems at the bottom, break 

 off and float to the surface, where they open and the pollen is 

 scattered. After pollination the flower-scape coils up and draws 

 the flower underneath the water, where the fruit is formed. 



6. Protection from water movement. This is provided for in a 

 number of ways, according to the velocity and force of the water, 

 and the habit of the plant. The pliant condition of the stems 

 and leaves gives them considerable mobility in the water, and 

 they are protected from injury which wave motion or currents 

 would inflict on a more rigid habit. Submerged leaves are usu- 

 ally small, or finely dissected. Many of the marine algae are 

 pliant and tough, so they resist the violent play of the water, or 

 the shock of the breakers against the rock. Postelsia, a marine 

 alga along rocky shores of the Pacific, has a leathery or rubber- 

 like consistency. There is an upright cylindrical stem 2 to 3 

 feet high, from the top of which hang strap-shaped leaves. These 

 plants with others of similar consistency grow near the edge of 

 the water, where they receive the full force of the breakers as 

 they come in shore. The plants are laid prostrate with each 

 thrust of the wave, and then recover as it recedes. Certain 

 fresh-water algae, as species of Lemanea, Cladophora, etc., grow 

 in streams where the water plunges over rapids or falls. Cer- 

 tain species grow only where the water is very violent, other 

 species of the same genera where it is less violent, and some 

 species of Cladophora as well grow in quiet water. 



953. Mesophytic structures. Those plants which during 

 the growing season are in general subject to medium or mod- 

 erate conditions of environment have been called mesophytes. 

 In the tropics, where the heat is great but rains abundant and 



